Printing with multiple colors or multiple shades of gray uses a halftoning process to convert continuous tone images to a printable format. For instance, 24 bit/pixel continuous tone image data may be converted into 3 or 4 bit/pixel print data. This allows the use of printing technology that imprints ink in fixed quanta (i.e. zero or more droplets per pixel.) With only a few actual colors being printable, the perception of a multitude of color tones is created by the combination of adjacent printed pixels.
Solid ink jet printers employ phase-change inks that are solid at ambient temperatures and liquid at elevated operating temperatures. These printers eject liquid phase ink droplets from the print head at a higher than ambient operating temperature. The droplets solidify quickly upon contact with the surface of the receiving substrate to form a predetermined pattern. For a direct to media printer, the substrate is the media. For an offset printer, the substrate may be an accumulator drum, blanket, transfer belt or other intermediate carrier from which the ink is subsequently transferred to paper or other print media in a fusing or transfix process. Among the advantages of solid ink is that it remains in a solid phase at room temperature during shipping and long-term storage. Problems with clogging in the print head are less prevalent than occur with aqueous based ink jet print heads. The rapid solidification or hardening of the ink drops upon striking the receiving substrates permits high quality images to be printed on a wide variety of printing media.
Solid ink printers may be designed to use any number of inks, but typically use four ink colors: cyan (C), magenta (M), yellow (Y) and black (K). Color images are formed on the receiving substrate by placing combinations of zero or more droplets of C, M, Y, or K ink at each pixel location. For example, if a green pixel is required, one drop of yellow ink is deposited with one drop of cyan. In most four color printers using CMYK colors, combinations of equal parts CMY or CMY plus any K droplets, placed together on one or distributed over a neighborhood of pixel locations, is perceived as approximately a neutral color where neutral is defined as perceptually near gray. This provides at least two ways to produce any approximately gray color when using a typical four-color ink set.
In a solid ink printer, output speed and printer resolution are determined by factors including droplet size, droplet spacing and number of droplets formed in the same location. Many printers support multiple user selectable output modes providing a range of output quality and print speed tradeoffs. For example, in draft or fast color mode, pages per minute are increased, usually at the expense of print quality. While draft mode may be achieved by changing the size of the droplets, increasing the spacing between droplets, decreasing the number of droplets or some combination thereof, in most ink jet printers draft mode is accomplished by increasing the droplet spacing to reduce the number of rows and/or columns that must be marked thereby increasing print speed. Increased droplet spacing reduces addressability and, without a corresponding increase in droplet size, decreases density. With decreased density, draft mode black may not be dense enough causing such print quality problems as poor black text and mottled look on black fills.